The present invention relates to semiconductor laser devices, and more specifically to a semiconductor laser device particularly suitable for oscillating rays of short wavelength.
A method including second harmonics generation (hereinafter referred to as "SHG") is known as a means to obtain laser rays of shorter wavelength. At present, the SHG using LN or KDP is realized. In this method, laser rays are focused to the LN or KDP by a lens thereby the SHG is generated. The SHG by Cherenkov radiation of a waveguide of a LiNo.sub.3 thin film using a semiconductor laser as a pump source is disclosed in CLEO (Conference of Lasers and electro-optics), pp. 196 and 199, WP6 or 0 plus E, 1988, vol. 103, pp. 93-97. Its structure is shown in FIG. 6. In FIG. 6, numeral 60 designates a LiNO.sub.3 substrate, numeral 61 designates incident semiconductor laser rays, numeral 62 designates a proton-exchanged waveguide, numeral 63 designates a transmitted fundamental wave, and numeral 64 designates second harmonics respectively.
On the other hand, it is known that the SHG is generated from the semiconductor laser itself. Since material to constitute the semiconductor laser has a large non-linear coefficient and the luminous density is large within the cavity of the semiconductor laser, this SHG is generated. Regarding the generation of the SHG by the semiconductor laser itself, for example, the Appl. Phys. Lett., vol. 3, pp. 52 reports the SHG generation in the GaAs semiconductor laser. In this case, since absorption of the SHG rays in GaAs is large, rays radiated from the laser facet in the vertical direction are limited to the rays generated in the region within 100 .ANG. from the laser facet, and rays generated in other region are absorbed within the GaAs.
Among the prior art as above described, in the former method, although the alignment of the optical system is important, this is very difficult in the actual state. Consequently, this method lacks reliability and the cost becomes high.
Also in the latter method, since the absorption of the generated second harmonics by the laser material is very large as above described, the output is about 10.sup.-5 times of that of the fundamental wave at most, and it is difficult to obtain a high output.